HR: 0800h
AN: S51E-1068    [Abstracts]
TI: Upper mantle shear and compressional velocity structures beneath southern Africa
AU: * Wang, Y
EM: yiwang1@ic.sunysb.edu
AF: Department of Geosciences, State University of New York, Stony Brook, NY 11794-2100 United States
AU: Wen, L
EM: lianxing.wen@sunysb.edu
AF: Department of Geosciences, State University of New York, Stony Brook, NY 11794-2100 United States
AU: Weidner, D
EM: Donald.Weidner@sunysb.edu
AF: Department of Geosciences, State University of New York, Stony Brook, NY 11794-2100 United States
AB: The velocity structures in the upper mantle play an important role in understanding mantle composition and temperature. In this study, we constrain the fine seismic shear and compressional velocity structures in the upper mantle beneath southern Africa by waveform modeling the seismic data recorded in the Kaapvaal array at the distance range of 9°-28° for an event occurring near Lake Tanganyika in east Africa. We then explore mineralogical models that would explain the inferred seismic structures. The seismic data recorded at this distance range provide excellent sampling of both the SH and P velocity structures in the top 800 km of the mantle. The first direct arrivals in both the P and SH data become weak at an epicentral distance of about 20°, indicating presence of a low velocity zone beneath southern Africa at a depth of about 150 km. In the SH data, the observed travel times of the reflected and triplicated phases off the 410-km discontinuity require a large shear velocity reduction in the low velocity zone and a small shear velocity jump across the 410-km discontinuity; the observed triplications at the 660-km discontinuity require a large shear velocity jump across the 660-km discontinuity. In the P wave data, the observed travel times of the triplication at the 410-km discontinuity suggest a small P wave velocity reduction in the low velocity zone, a large velocity jump across the 410-km discontinuity and a high Vp/Vs ratio in the transition zone; the triplication at the 660-km discontinuity is indiscernible, suggesting a small P wave velocity jump across the 660-km discontinuity. Overall, the seismic data can be explained by a 150-km thick high-velocity lid overlying a low velocity zone between 150 km and 405 km depths with a P wave velocity reduction of -1.5% and an SH wave velocity reduction of -9%, followed by a small shear velocity jump of 3% and a large P velocity jump of 10% across the 410-km discontinuity, a transition zone with a high Vp/Vs ratio, and a large shear velocity jump of 9% and a small P velocity jump of less than 4.5% across the 660-km discontinuity. Although the velocity reductions in the low velocity zone trade off with assumed depth of the 410-km discontinuity, the seismic P and SH data require the existence of a low velocity zone beneath southern Africa. In joint modeling of the mineral physics and seismic data, we explore a variety of compositional and thermal models by comparing their predicted velocity profiles using the Tr660 program developed by Weidner and Wang (1998) with the seismic velocity structures inferred from the seismic data. We will present a best-fitting mineralogical model with its predicted velocity structure best explaining the seismic data.
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 3947 Surfaces and interfaces
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: Fall Meeting 2005